Proper chromosome segregation is essential for normal cell division and development. In multi-cellular organisms, the microtubule-based bipolar spindle is a large complex cellular machine responsible for correctly separating and partitioning chromosomes during cell division. In higher eukaryotes, spindle structure and function must constantly tailor to meet the requirements imposed by unique cell divisions within a single organism. For example, during meiotic divisions spindles have rounded poles and are acentrosomal lacking prominent astral microtubules whereas mitotic spindles generally have focused poles with centrosomes that emanate astral microtubule arrays. It has been shown that spindles assembled in meiotic and mitotic extracts morphologically resemble their in vivo counterparts, which suggests that there are intrinsic factors determining spindle structure. How spindles within a single genetic background become modified under various conditions in order to meet specific functional requisites is not well understood. The experiments proposed here will investigate distinct structural differences between spindles of different cellular origins but identical genetic backgrounds, specifically meiotic versus mitotic. The central hypothesis is that meiotic and mitotic spindles differ in molecular composition and regulation. To understand spindle differences, we will focus on investigating structural and molecular differences of spindle poles and centrosomes through three specific aims. Through the proposed aims we will determine whether cytoplasmic factors specify spindle pole structure, examine how and which spindle pole and centrosome factors are differentially regulated in meiosis versus mitosis, and how changing spindle pole structure and molecular composition alters overall spindle function and dynamics. We will employ the advantages of the unique model system Xenopus laevis, by using a combination of in vitro and in vivo approaches and manipulations that will be complemented with in silico modeling. The proposed research will elucidate the underlying molecular mechanisms contributing to functional and structural spindle differences that will advance our understanding of chromosome segregation and cell division, which is a crucial process involved in many human diseases and cancer. Therefore, the proposed work is significant, as the results from this study will improve our understanding of basic cell biology an the progression of human disease.